The leech has always been a subject of great interest to medical historians and collectors alike. The concept of an imbalance of humors or fluids within the body dates back to early Chinese texts and is followed by numerous other medical movements. Galen in particular is remembered for his concepts of the four humors: blood, yellow bile, black bile and phlegm. His concepts that were developed from his predecessors, especially Hippocrates led to the natural consequence of the belief that bleeding could restore this balance and persisted well into the 19th century. The leech proved to be a very efficient method of bleeding.
The diagram of a leech shown in the next page is taken from Johnson: A Treatise on the Medicinal Leech. Reference 216.